Countless studies have hitherto been made in the efforts of reducing noise of electric blowers for use in vacuum cleaners and the like apparatuses. One example of such electric blowers is to generate an air output by converting a dynamic pressure obtained by centrifugal force of rotary fan 105 into a static pressure with an air guide.
FIG. 10 is a sectional view illustrating a conventional electric blower. Electric blower 150 shown in FIG. 10 comprises stator 101 and rotor 102 mounted on bracket 103 as an electric motor. Rotor 102 has rotary fan 105 mounted to one end of output shaft 104 that projects from bracket 103. There is air guide 106 disposed as a partition for separating between rotary fan 105 and the electric motor. Rotary fan 105 mounted to output shaft 104 is rotated to produce a flow of suctioned air from opening 105b when the electric motor is driven. This airflow is deflected to a radial direction 90 degrees from an axial direction, and flows outward in the radial direction while gaining a dynamic pressure given by fan blades 105d of rotary fan 105. The airflow delivered from rotary fan 105 is decelerated as it passes through an airflow path composed of diffuser 106a of air guide 106 disposed around the outer periphery of rotary fan 105, and converted from the dynamic pressure into a static pressure. After having passed through diffuser 106a, the airflow is forced to change its direction for 180 degrees in the way to pass through return path 109b composed of the outer periphery of air guide 106 and cylindrical portion 108d of fan case 108. The airflow is further guided into the electric motor by guide vane 106b of air guide 106 through partition plate 106c, and blown to the outside while cooling the electric motor.
Fan case 108 has a shape as shown in FIG. 10, which comprises fan-facing portion 108c, fan case shoulder 108b and cylindrical portion 108d. Fan-facing portion 108c is formed to face rotary fan 105 and extend radially about air inlet opening 108a. Fan case shoulder 108b is curved from the outermost part of fan-facing portion 108c to become parallel with output shaft 104, and cylindrical portion 108d extends cylindrically in parallel with output shaft 104 from fan case shoulder 108b. Fan case shoulder 108b is provided with a fillet of large circular arc formed to make the airflow turn around for 180 degrees after it passes diffuser 106a. Here, the fillet refers to a rounded shape so processed by joining two surfaces with another piece having an arc shape in cross section. In addition, a corner at an exit side in the airflow path of diffuser 106a is also cut to form an arc shape in a manner to conform to fan case shoulder 108b, and designated as diffuser shoulder 106e. 
FIG. 11 is a drawing that schematically illustrates shapes of fan case shoulder 108b and diffuser shoulder 106e of the conventional electric blower. In FIG. 11, the shapes of fan case shoulder 108b and diffuser shoulder 106e are depicted in their meridian plane. In other words, FIG. 11 represents a sectional view of fan case shoulder 108b and diffuser shoulder 106e as they are cut with a plane containing output shaft 104, and that this sectional view includes a revolved projection of diffuser shoulder 106e. As shown in FIG. 11, both fan case shoulder 108b and diffuser shoulder 106e of the conventional structure have circular arc fillets formed to have radius R. These conventional fan case shoulder 108b and diffuser shoulder 106e have circular arc to radius ratios of the same value.
As a method of designing an electric blower of this type, an inner diameter, an outer diameter, an inlet opening height and an outlet opening height of each of the rotary fan and the air guide are determined according to working points such as a flow rate, a pressure and a rotating speed of an electric apparatus for which the electric blower is used. In addition to these factors essential for the designing, it is also necessary to form an airflow path of the shape capable of reducing abrupt changes in the pressure and flow velocity in order to achieve noise reduction of the electric blower. This is for the purpose of making it capable of suppressing development of turbulent airflow. There are other measures taken for this purpose such as an improvement in the shapes of individual parts of the air guide in addition to designing the shape of fan blades (refer to patent literatures 1 and 2, for example), an idea of reducing changes in the pressure that occur when trailing edges of the rotary fan blades cross a leading edge of the diffuser by increasing a distance between the trailing edges of the rotary fan blades and the leading edge of the diffuser, decreasing a rotating speed of the rotary fan, and so on.
The method discussed above to increase the distance between the trailing edges of the rotary fan blades and the leading edge of the diffuser gives rise to a drawback, however, that it increases a loss attributable to increase in slippage and back-flow of the air at the trailing edges. In addition, the efficiency of blowing air also decreases due to a decrease in the dynamic pressure when the rotating speed of the electric blower is reduced.
There are also other means to achieve noise reduction by disposing a soundproofing material, a noise attenuation mechanism, and the like in a main body of an apparatus such as vacuum cleaner. However, these means also reduce a suctioning power of the vacuum cleaner and worsen the operability since they lead to an increase in pressure loss inside the airflow path as well as an increase in weight of the main body of the apparatus.
PTL 1: Unexamined Japanese Patent Publication No. 1986-40495
PTL 2: Unexamined Japanese Patent Publication No. 2005-220853